Amazon.com announced this week that it’s now selling more electronic books through its Kindle e-readers than it is selling printed books.

Kindle book sales are up three-fold so far this year, and Amazon’s combined U.S. Kindle and printed books business is now growing faster than it has in 10 years, when sizzling growth was easier to post because the previous year’s base was much smaller than it is now.

So we have our answer on whether e-books are good for book sales, a question that bedeviled the publishing industry around the Kindle’s 2007 introduction. It’s a little bit more complex a question to ask whether it’s good for authors, given the different economics of what writers get paid for hardcover, paperback and electronic books.